With the rapid development of electronic, communication and computer industries, mobile electronic communication equipments, such as, for example, camcorders, mobile phones, laptop computers, and the like, have been advancing remarkably. Accordingly, the demand for lithium secondary batteries as a power source of mobile electronic communication equipments is increasing day by day. In particular, recently, research and development has been actively made all over the world including Japan, Europe, USA as well as Korea, in relation to the applications of lithium secondary batteries not only as a power source of mobile electronic equipments but also as an environment-friendly power source of larger scale equipments, for example, electric vehicles, uninterruptible power supplies, electromotive tools, satellites, and the like.
Generally, a lithium secondary battery includes a cathode of lithium-transition metal composite oxide, an anode capable of intercalating or disintercalating lithium ions, a separator interposed between the cathode and the anode, and an electrolyte that helps the migration of lithium ions.
The main function of the separator is to holding the electrolyte therein to provide high ion permeability as well as isolating the cathode from the anode. Recently, a separator having a shut-down function has been suggested, in which, for example, when a short circuit occurs in a battery, a part of the separator melts to stop pores so as to keep a large amount of electric current from flowing into the battery. Also, techniques have been suggested to prevent cathode and anode plates from coming into contact with each other by increasing an area of a separator larger than those of the cathode and anode plates, however, in this case, an additional function is required to keep an internal short circuit of a battery caused by the contact of the two electrode plates from occurring by preventing a separator from shrinking due to an increase in internal temperature. In particular, to keep up with the recent trend of a lithium secondary battery toward higher capacity and higher energy density, higher heat resistance and thermal stability than required for a conventional separator is required because temperature in a battery rises when a high rate charge/discharge state of the battery is continuously maintained.
In the conventional separator, a porous membrane made in a form of a sheet using polyolefin-based polymer such as polyethylene (PE) or polypropylene (PP) has been widely used. A separator made from polyethylene having a melting temperature of 130° C. or polypropylene having a melting temperature of 170° C. stops micropores to block (shut down) the movement of ions in response to heat generated when an excessive amount of electric current flows into a battery by a short circuit or an increase in the internal temperature by the effect of a certain external factor, and along with this, thermally shrinks or melts to fulfil a separation function.
In addition to the shut-down feature, shape stability is another important quality of the separator when the temperature continues to increase even subsequent to shut-down. When used in the melting temperature range of polyethylene or polypropylene, if the temperature of the battery continues to increase by internal/external factors even subsequent to shut-down, the separator melts and loses its shape, which causes a short circuit of electrodes, getting into a dangerous situation.
To solve this problem, development of a composite separator for a lithium secondary battery has been reported. First, Korean Patent Publication No. 10-2011-0011932 improves thermal stability by coating ceramic responsible for enhancing mechanical properties and ionic polymer on polyolefin-based resin.
Also, Korean Patent Publication No. 10-2010-0129471 increases a mechanical strength and provides acid resistance by coating with inorganic powder, thereby improving performance and life characteristics. However, this art involves coating with an inorganic matter which increases in the separator weight and consequently the overall battery weight, causing a reduction in energy density. Also, because inorganic fillers vulnerable to reaction with a majority of carbonate are used, CO2 gas and other gases are generated by decomposition of a carbonate-based solvent being commonly used, and when left unused for a long time, a bulging phenomenon of a cell is observed, hence there is a limitation in improving battery stability.
PCT/EP2003/007163, which provides large inorganic particles to the surface and inside, reduces a density increase of a hybrid separator and when it reaches a high temperature, brings about shut-down by penetration of an inorganic matter into pores, but improves safety due to having no melting point. However, this also has a factor hindering stability because gas is generated by a decomposition reaction of the used inorganic matter, that is, Si-based, with an electrolyte solution.
Recently, many attempts have been made to coat and graft polyvinylidene fluoride (PVdF) onto a polyolefin-based porous membrane by a method of double-coating polymer. However, due to low heat resistance, polyolefin melts and micropores disappear when an internal temperature of a battery exceeds 150° C., and thus, an ion blocking effect is superior but when a microporous polymer membrane melts, a membrane area reduces due to a very high volume shrinkage ratio, as a consequence, an internal short circuit of a battery occurs, causing a problem with safety of the battery.
Japanese Patent Publication No. JP-P-2007-122026 manufactures and uses a laminated porous film by coating a molten thermoplastic resin in a form of a non-woven fabric onto a porous film, and this provides excellent thermal safety, but the non-woven fabric form is exposed to an internal short circuit hazard, and due to high porosity, a large amount of electrolyte solutions needs to be used, resulting in a reduced energy density.